Project Summary Our long-term objective in the proposed experiments is to obtain new knowledge on the basic mechanisms that control retinal development and to apply this knowledge to develop novel ways to treat retinal degenerative diseases. Our strategy is to use genetically engineered mouse models that we have already created or that we will create. Although an impressive amount of information has accumulated on the mechanisms that control retinal development, large gaps still remain. In particular, the mechanisms that control a progenitor cell's decision whether to proliferate or differentiate are only vaguely understood. A better understanding is of great importance for finding new ways to repair damaged retinas. Retinal ganglion cells (RGCs) are the first cell type to differentiate from retinal progenitor cells (RPCs) during development and are the retinal neurons that connect to the brain. We focus on the regulatory events that cause RPCs to commit to a RGC fate. In related experiments, embryonic RPCs will be used to repopulate RGCs in adult retinas that have been depleted of their endogenous RGCs. Our underlying hypothesis is that for RPCs to differentiate into RGCs, Atoh7 must integrate with other regulatory factors to achieve a balance between proliferation and differentiation. To address the hypothesis, we proposed three specific aims. The first aim will determine whether Atoh7 is sufficient to convert non-RGCs to a RGC fate. Preliminary work indicates that replacing Neurod1 with Atoh7 leads to ectopic RGC gene expression in the inner nuclear layer. We will determine whether Atoh7 can drive RGC differentiation in non-RGC neurons in developing and adult retinas. The second aim will determine whether Atoh7 regulates Notch signaling to control the balance between RPC proliferation and RGC commitment. In preliminary experiments, we found that Atoh7 binds to E-box elements upstream of Notch1 and that Atoh7 negatively regulates Notch1 expression. We will identify the time at which Atoh7 appears relative to Notch signaling. We will determine whether Atoh7 and Notch1 participate in a negative feedback loop and whether RPC proliferation is perturbed when the Atoh7 binding sites on Notch1 are deleted. In the third aim, we will optimize our experiments on repopulating RGC-depleted retinas by transplanting Atoh7-expressing RPCs into the retinas of RGC-depleted mice along with neuroprotective factors. We will also determine whether Atoh7-expressing RPCs can regenerate optic nerves in optic nerve crush and other mouse models. Our knowledge of the factors controlling retinal development allows us to apply developmental concepts to adult retinas. We have developed realistic genetic models for human optic nerve degeneration that will have ultimate use in stem cell replacement therapy to repair damaged optic nerves.